The present invention relates to a steam generator with an evaporation tank and at least one heating body.
Steam generators are, for example, employed for air humidification and generally comprise an evaporation tank which is partially filled with water. The water is heated and evaporated by means of heating bodies, such as electrodes or electrical resistances, for example, in which process mineral salts are precipitated which have to be removed from time to time from the evaporation tank by blowdown. For this purpose, in order to top up with water and, in some cases, also to monitor the water level by means of a float in a float tank outside the evaporation tank, the latter has at least one water passage opening, which is frequently arranged in the bottom of the evaporation tank.
Particularly when water with a very high lime content is employed, a major problem is the formation of limescale and the rapid closing up of the usually relatively small water passage opening or openings, which often makes cleaning necessary after only a short period. If larger water passage openings are employed, the blowdown quantity can scarcely be controlled with respect to time and the danger exists that the water in the evaporation tank will start to oscillate shortly after the blowdown.
In order to keep the time intervals between the cleaning operations as large as possible, attempts have been made to provide the water passage openings with devices which prevent rapid blockage. Known arrangements are, for example, the use of screens with a plurality of suitably arranged openings and the pumping of air through the openings from below in order, by this means, to keep water paths free. Also known are openings which are cleaned by moving parts. The solutions proposed, however, do not permit the maintenance cycles to be increased in a satisfactory manner for all types of water.
In view of the problem, with certain types of water, that the water passage openings rapidly form limescale in the previously known steam generators, the invention is based on the following object. A steam generator is to be created with an evaporation tank with at least one water passage opening which does not form limescale or, at least, forms limescale more slowly than the water passage openings of the previous steam generators.
This object is achieved by means of the steam generator according to the invention, as defined in the independent claim 1. Preferred embodiment variants are given in the dependent claims.
The essence of the invention consists in the fact that, in a steam generator with an evaporation tank and at least one heating body, the evaporation tank has an at least partially double-walled design with an inner wall and an outer wall which bound an outer chamber. The inner wall has at least one inner water passage opening connecting the inside of the tank to the outer chamber and the outer wall has at least one outer water passage opening, so that the water supply to the inside of the tank and the water withdrawal from the inside of the tank take place by means of the outer water passage opening or at least one of the outer water passage openings, the outer chamber and the inner water passage opening or at least one of the inner water passage openings.
The inner water passage opening or water passage openings, which are subjected to the hot water, which is precipitating lime, on the inside of the tank can be selected as relatively large openings so that they do not close up too rapidly. The water supplied from the outside, which penetrates through the at least one outer water passage opening into the outer chamber and from there through the at least one inner water passage opening into the inside of the tank, is heated so little during this time that practically no lime is precipitated in the outer chamber. In consequence, only relatively little lime passes to the outer water passage opening or openings so that these can be selected as smaller openings. In this way, the maintenance cycles of the steam generator can be increased without, in the process, having to make concessions with respect to the control of the blowdown necessary from time to time or to the stability of the water level in the evaporation tank.
In a preferred embodiment variant, the evaporation tank comprises an evaporation vessel, which has an opening at the bottom and, arranged underneath it, an exchangeable lime receptacle with a filling opening, so that lime precipitated inside the evaporation vessel passes through the evaporation vessel opening and the filling opening into the lime receptacle. This avoids the inside of the evaporation vessel being filled with lime and the lime precipitated does not have to be removed so rapidly. In addition, removal of the lime is simpler, particularly when the lime receptacle is replaced or emptied before it is completely full of lime.
The double-walled part of the evaporation tank can be located either on the evaporation vessel or on the lime receptacle or on both.
It is advantageous for an approximately horizontal or outwardly inclined lime deposition surface, which can be formed, in particular, by the inside of a part of the outer wall, to be arranged in the outer chamber. This lime deposition surface captures lime particles which flow through the at least one inner water passage opening into the outer chamber, and the lime released in the outer chamber itself, even before the outer water passage opening or openings.